Impact of multi-day field calibration of novel cosmic-ray soil moisture sensors

Soil moisture plays an important role in agriculture, hydrology, and land-atmosphere processes. Over the years, there has been a scale gap between traditional point-scale sensors and large-scale soil moisture products from satellite remote sensing. Recent advances suggest the use of cosmic-ray sensors (CRS) that provide integrated soil moisture at subkilometer scale (~300m radius and 10 to 70cm depths). The CRS works based on interactions between cosmic-ray neutrons which are attenuated by hydrogen content within its footprint (mainly soil moisture), so that more (few) neutron counting rates by the sensor corresponds to drier (wetter) soils. Site-specific calibration is required for this method, which is based on collecting soil samples within the footprint of sensors. Up until recently, it has been accepted that a single calibration day would provide enough information for accurately calibrating. However, recent modeling studies suggest that multiple calibration days can reduce CRS signal uncertainty. For this research, we test the impact of multiple-day calibration at three sites located in an organic farm in southern UK. We have analyzed all possible combinations from 3 calibration days to investigate whether uncertainties are further reduced with additional days as well which wetness combination can provide best accuracy to CRS. High uncertainties were observed when only one single soil sampling was taken. The results also indicated the benefits of at least 2-day calibrations at all sites which gives optimum soil wetness condition at approximately 0.4cm³cm^-3.